Various types of optical recording media such as CD-R, CD-RW, MO and so froth are widely recognized and spread as external storages for information processing apparatuses such as computers because they can store a large volume of information and can be randomly accessed easily. With an increase in quantity of handled information, there is a demand to increase the recording density.
Among various optical recording media, optical disks having a recording layer containing an organic dye (also referred to as a dye containing recording layer) such as CD-R, DVD-R, DVD+R and the like are particularly widely used because they are relatively inexpensive and have compatibility with read-only optical disks.
Media such as CD-R representative of optical disks having the dye containing recording layer, for example, are in a laminated structure which has a dye containing recording layer and a reflective layer in order on a transparent disk substrate along with a protective layer for covering the dye containing layer and the reflective layer. Recording and reading are performed with a laser beam through the substrate.
DVD-R (single-sided, single-layer DVD-R), which is representative as well, has a laminated structure in which a dye containing recording layer, a reflective layer and a protective layer covering them are formed in this order on a first transparent disk substrate, and a so-called dummy disk, which has a second disk substrate (which may be transparent or opaque) and, if necessary, a reflective layer formed on the second disk substrate, provided on the protective layer through or not through an adhesive layer.
Recording or reading are performed with a laser beam from one side of the disk through the first transparent disk substrate. The dummy disk may be of only a transparent or opaque disk substrate, or may be provided with a layer other than the reflective layer.
Meanwhile, DVD+R has almost the same structure as DVD-R, description of which will be hereinafter represented by DVD-R.
In order to largely increase the recording capacity of the optical recording medium, two single-sided DVD-Rs as above are bonded together to form a medium having two recording layers, which is known as a double-sided DVD-R (double-sided, dual-layered DVD-R). Recording and reading are performed by irradiating a laser beam onto each of the recording layers from the both sides (that is, the laser beam is irradiated from one side of the medium to perform recording and reading on a recording layer closer to this side, while the laser beam is irradiated from the other side of the medium to perform recording and reading on the other recording layer closer to the other side).
With respect to optical recoding media having a plurality of recording layers, there is, in these years, a demand for a single-sided incident type optical recording medium (for example, single-sided incident dual-layered DVD-R) on which recording and reading can be performed on a plurality of recording layers by irradiating a laser beam from one side so as to avoid an increase in size and complexity of the recording/reading apparatus, enable continuous reading from the plural recording layers, and improve the facility.
To meet the above demand, there has been proposed a single-sided incident type DVD-R of the dual layer type (single-sided, dual-layered DVD-R) having two recording layers, for example, as a single-sided incident type optical recording medium having the structure below (refer to Japanese Unexamined Patent Publication No. HEI 11-066622, for example).
For example, a single-sided incident type DVD-R of the dual layer type of the bonded type is formed by laminating, on a first light-transmissible substrate, a first recording layer made from an organic dye on which information can be optically recorded by irradiating a laser beam for recording, a first reflective layer made of a semi-light-transmissible reflective film that can pass through a part of the laser beam for reading, an intermediate layer that can pass through the laser beam for recording and the laser beam for reading, a second recording layer made from an organic dye on which information can be optically recoded by irradiating the laser beam for recording, a second reflective layer reflecting the laser beam for reading, and a second substrate in this order.